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1975 R75 /6 Grounding Cable

I'm riding along going to the gas station that sells non-ethenonl fuel when my bike dies. I pull off and immediately notice the grounding wire dangling next to the bike. The end had broken that was bolted to the engine case. I pushed the bike to a parking lot and tried to reconnect what was left of the end of the grounding cable. The bike would start and then the generator light would come on and then it would die. I went to an auto parts store and bought a cable that was made for a car so the eyelets in the end of the cable were too big and I'm thought that it should have been grounded well enough, but the same symptoms occurred. I then suffered the humiliation of having it towed home (but my neighbors thought it was a new bike).

Other than getting a proper ground wire and testing the battery (1 yr old), are there any suggestions of what else could be wrong? Could I have damaged the generator somehow?

I did put a battery tender on right when I got home and the light on the battery tender that evening turned green.

I don't think you've damaged your charging system...you can run the bike with a non-charging system (red light on) for as long as the battery holds out. More than like there's something wrong with the connection at the speedo cable boot. That bolt can strip quite easily if over torqued. You can use any other grounding point, say one of the bolts that holds the tranny cover on. You might have trouble fitting the thickness of the cable eyelet onto the stud and still get the nut to engage. But any engine bolt that you can easily find and attach to should be good for temporary...even long term of you do it right.

Ok, I finally got around to replacing the grounding cable. Had the tank off the bike and started it with no problem. Put the tank back on and no ignition lights. Took the tank back off and started fine. I had the ignition on and jiggled the wire going from the battery to the starter and the ignition lights flashed on and off. I replaced this wire and secured all connections, put the tank back on started the bike and went for a ride.

Right out of the gate, the generator light came on very dimly and the bike lost power for just a second (this happened twice but i figured that I may have run down the battery with all of my testing) after riding for 10 min or so around the neighborhood all seemed normal and I took it out on the road. 15 mi, no problems. on my way back down the last straight away, I poured on the gas to pass a car. The generator light came on and the bike lost power, i let up on the gas and it ran fine. It did this again each time I would pour on the gas, the generator light would come on and I would lose power until i let off the gas.

Any ideas would be greatly appreciated. The battery is less than 1 yr old, but I have not tested it.

IMO, you have entered into the nebulous land of poor electrical contacts. I trust you can figure out how to use a mulitmeter and measure voltages? If not, you will need to learn.

A technique I found especially useful is to look for voltage drops at wiring connections. That is how I found out (with much disappointment) that my ignition switch was faulty. The basic approach is to put one end on ground (engine) and probe with the other, starting at the battery. The battery should, depending upon whether lights are on, give a voltage between 12 and ~12.6V.

That battery voltage is then a reference. The other locations, i.e. starter, coil power, ignition switch, etc. are then measured and the readings compared to the battery voltage. Much over 0.5 V difference indicates a problem.

I did not think this up myself, instead there is a really good electrical system diagnostics book at the Chicago BMW owner club website: http://crbmw.com/rokcart

It is the best 'manual' on electrical system troubleshooting I have found. I have read that Oak wrote or co-wrote most of the manual, which is intended to 'school' you on BMW motorcycle electrics. It has some good information and ideas for basic troubleshooting. Good luck!

From the symptoms you describe (not from my actual experience), what about a rotor that is going bad? It holds together at lower RPMs, but then the force of the higher revs makes it open up (electrically)?

From the symptoms you describe (not from my actual experience), what about a rotor that is going bad? It holds together at lower RPMs, but then the force of the higher revs makes it open up (electrically)?

For whatever reason (probably failed rotar) your generating system is not charging battery

You have electronic ignition - these systems have a definite voltage requirement below which
they will not trigger and generate a spark

You start out ok with marginally charged battery and run for a while and as bat voltage declines
it finally shuts you down

When you get on the throttle your bike demands more sparks/second from the elec ignition and it
fails - when you let off the throttle the demand is less and you can run a bit farther

Eventually the voltage falls enough to shut you down completely - if you let it sit for a while
maybe you can restart and go a bit farther at moderate speed but you must fix the charging
system before you can count on anything useful

Ok, I load tested my battery. Test OK. with the bike off, the battery voltage was about 12.8. Bike starts. At about 3k RPM, the volt meter reads almost 14v. At about 3k RPM and over, the GEN light comes on and off dimly and the bike engine feels like it is running erratically. I was told to check the red cable on the starter relay. Any other meaningful advice would be greatly appreciated.

Check and/or replace the wires from the stator to the diode board assembly. These are the U, V, W wires labeled on the stator and route behind the diode board. If these wires look suspect, replace them. They probably need replacing if they are original because of the engine heat and wire routing.

The next thing is to get a VOM and check the voltage at the green coil wire - with the ignition on - and compare that voltage to the battery positive. The green wire supplies voltage to the coils and points/ignition module. If the difference in voltage of the battery positive terminal and coil power is more than 1/2 volt, there are other problems in the wiring. These same voltages should be checked with the engine running. Good luck!

It would be helpful to see if the engine will run smoothly past 3k without the charging system operating at all
to know if problem is caused by charging system/voltage or some other ignition or carb problem that crops up at 3k

With fully charged battery and wires removed from alternator rotar brush terminals and maybe headlight bulb
removed - Does the engine spin up as it should or still falter?

Im with Stan.. check the diode board

I would also check the coils and your electronic ignition system. Clean and check all connection points. You might see what happens with a stock system while you sort all of this out.
It is a by the numbers thing.. top to bottom.... you will find the issue.

Well, Rick nailed it. There was a loose wire on the starter relay. Made sure all the wires were plugged in and she fired up and ran like it was new. Thanks to everyone for your insight and help. I did notice that the female spade ends on the wires connecting to the starter relay do not have a real snug fit on the mail ends on the relay. Is there something that I can do to snug the ends onto the relay?